- further information: DirectShow Playback Guide
Video files include their audio tracks as a separate "stream" contained inside the file. This audio stream can be in a wide variety of formats, but it is very commonly one of three options: uncompressed PCM (WAV audio essentially), MP3, or AC3 (Dolby Digital). In order to properly play back the audio track you will need to have a decoder that can handle the proper audio type installed and configured on your system.
Quite often, when the audio fails to play back it's because the audio track is in AC3 format. If you are using FFDSHOW (or CCCP) to handle the decoding of your video files, FFDSHOW can also handle decoding of AC3 content all on it's own, but you need to enable the option. Simply open the FFDSHOW Audio Decoder Configuration dialog (Start --> Programs --> FFDSHOW or Combined Community Codec Pack --> FFDSHOW Audio Decoder Configuration) and choose Dolby Decoder from the tree on the left and enable it.
Another option is to use the standalone AC3filter, which also works quite well and reliably.